How to Form a Catalyst 3850 Data-Stack and Power-Stack

How to Form a Catalyst 3850 Data-Stack and Power-Stack

Introduction

This document describes the process to properly form a stack-wise and power stack with Cisco Catalyst 3850 switches. It also shows the use of some commands that will help to troubleshoot why a stack is not forming properly.

Requirements

In order to be able to form a switch stack more than 2 switches must be used.

The IOS version used must be the same for all switches. (Software auto upgrade can be configure on a existing stack in order to automatically upgrade newly added switches).

The same mode must be used on all the switches INSTALL or BUNDLE mode.

The right type of cables must be used to achive stack completion. Be aware that cables for the 3750 platform do not work on the 3850 platform.

Components Used

Cisco Catalyst 3850 switches.

Cisco 3850 Stack wise cables.

Cisco 3850 stack power cables.

Network Diagram

The example below, shows how to properly connect a stack of six 3850 switches, together with a power-stack connection example.

Note: The process will be similar for stacks of 2,3,4,5,7,8 and 9 switches.

Each switch has two stack-wise ports(left), let's name the stack wise ports P1 and P2 as shown in the picture.

switch_1 P1 connects to switch_2 P2 (Green cable)

switch_2 P1 connects to switch_3 P2 ( Orange cable)

switch_3 P1 connects to switch_4 P2 (Purple cable)

switch_4 P1 connects to switch_5 P2 (Violet cable)

switch_5 P1 connects to switch_6 P2 (Yellow cable)

switch_6 P1 connects to switch_1 P2 (Brown cable)

Stack of six C3850 Switches.

Picture 1 Switches in a stack.

Below the steps that show how to cable the stack for power-stack configuration:

StackPower is a feature that allows the power supplies to share the load across multiple devices in a stack. By connecting the switches with power stack cables, the power supplies of up to four stack members can be managed as a one large power supply that provides power to all switches and to the powered devices connected to switch ports.

Each switch has two power-stack ports, lets call the ports A and B ( A top, B bottom)

Let say for example, based on the diagram above. I would like to simulate a scenario wherein if I unplugged the data stack cable from Switch 1 Port 1 to Switch 2 Port 2. So even if the stack cable was removed, the show switch output should still be as follows (Switch 1: Active, Switch 2: Standby and the rest are members) assuming the priority is hard coded for each switch Priority 15 for Switch 1, Priority 14 for Switch 2, so on and so forth. And no traffic drop will occur. and the standby and member switches will not reboot?

If you remove the cable going from switch 1 port 1 to switch 2 port 2 the stack will remain in the same state as before removing the cable (This will only be true as long as there is another cable on switch 1 port 2 connected to another switch to ensure redundancy). As you mention, The active and standby switches will remain the same and none of the switches will reboot nor traffic drop should occur.

Thank you for your answer. Last question, let say in a 4 switch stack ring. Switch 1(active), Switch 2(standby), Switch 3 and Switch 4 (member). Their switch priority are 15,14,13, and 12 respectively. If switch 1 failed, switch 2 will become active, switch 3 will be the standby and switch 4 will stay as member. And if switch 1 went back online, the switch 1 will not preempt, and should become as member. Is there any command to force the switch 1 to become active after it went online?

Sorry for my late response. As of now, there is no way to make the previous master switch preempt. That switch will be elected as master again once there is a stack or the current active and standby fail.

Yes, stacking different models of 3850 switches is possible. However, when using the WS-C3850X models they need to be running at least version 3.7 in order to properly form the stack. Apart from that they stack should form properly.

Hi there appreciate if you could help ,i have just configured BFD between ASR 9K and an HP-5940.I have configured 10 links with a port-channel on the ASR 9K (Version 6.0.2) and 10 links BrIdge-AGG on the HPI am getting an error on one of the 1...
view more

Hi All,
I'm looking at using either a WS-C3850-12XS or a C9500-16X switch for an internal layer 3 node which will be used as a Route redistribution device between BGP and OSPF. I've found in the past that Cisco often limit the number of BGP r...
view more

Hello everybody,
I have got a problem with editing User Label from Cisco UCS Servers by UCS Central.
We want to store the ASSETID in the Userlabel field of an UCS Rackserver, Bladeserver or Chassis Userlabel field.
I can edit the value in UC...
view more